The present invention relates generally to the field of diagnostic radiography and, more particularly, to an anti-scatter X-ray grid device and a method of making the same.
Anti-scatter grids are widely used in X-ray imaging to enhance image quality. X-rays emitted from a point source pass through a patient or object and are then detected in a suitable X-ray detector. X-ray imaging works by detecting the intensity of X-rays as a function of position across the X-ray detector. Darker areas with less intensity correspond to regions of higher density or thickness in the object, while lighter areas with greater intensity correspond to areas of lower density or thickness in the object. This method relies on X-rays either passing directly through the object or being totally absorbed. However, X-rays may also undergo scattering processes, primarily Compton scattering, in the patient or object. Such X-rays generate image noise and thus reduce the quality of the image. In order to lessen the impact of such scattered X-rays, an anti-scatter grid is employed. The grid preferentially passes primary X-rays (those that do not scatter) and rejects scattered X-rays. This is done by interleaving materials of low X-ray absorption, such as graphite or aluminum, with layers of high X-ray absorption, such as lead or tungsten. Scattered X-rays are then preferentially stopped before entering the X-ray detector. However, a fraction of primary X-rays are also absorbed in the grid.
One of the primary metrics for anti-scatter grid performance is the quantum improvement factor (QIF), wherein QIF=Tp2/Tt. Tp is the primary X-ray transmission through the grid and Tt is the total transmission. This equation shows the importance of achieving a high primary transmission. If primary X-rays are lost, imaging information is also lost and thus either the X-ray dose must be increased or a degradation in image quality accepted. A QIF of 1 or greater indicates an improvement in image quality, while a QIF of <1 indicates that the grid actually harms the quality of the image.
The principal design metrics for an anti-scatter grid are the line frequency, the line thickness, and the grid height, often expressed as the ratio. The line frequency, typically expressed in units of lines/cm, gives the number of absorbing strips of material in a given distance. The line thickness is just the thickness of the absorbing lines, often expressed in units of microns. The grid ratio is the ratio of the grid height to the interspace distance (the amount of low-absorbing material between a pair of grid lines). Grid performance is also influenced by the material used in manufacturing the grid and the type and thickness of grid covers, which are non-active sheets encasing the grid to provide mechanical support.
In designing an anti-scatter grid, the degree of scatter rejection must be balanced with the primary transmission in order to maximize the quantum improvement factor. However, this is not always possible because of manufacturing limitations. For example, in a low-energy procedure, such as mammography, the grid lines are often thicker than required because of limitations in manufacturing grids with very thin lines. Moreover, in such low energy procedures, the interspace material can be a significant absorber of primary X-rays.
Traditional methods of grid manufacture involve laminating lead foils onto interspace material or using a fine saw to cut grooves in a graphite substrate and filling the grooves with lead. Molding has also been suggested as a method of grid manufacture, for example as disclosed in U.S. Patent Publication Number US20090272874.
Accordingly, there is an ongoing need for improving upon existing X-ray grid design and manufacturing techniques.